A multi-antenna wireless communication system may use multiple antennas for transmission, reception, or both. This use of multiple antennas is generally intended to improve one or more aspects of system performance, such as spectral efficiency or signal quality, for example.
During transmission with multiple antennas, the same signal is usually emitted from each antenna with slightly different signal properties (e.g., different phase and gain), which may result in favorable patterns of constructive and/or destructive interference. The differences in signal properties can be controlled through the use of precoding, a technique that modifies the signal for each antenna based on a precoding matrix.
The precoding matrix is typically selected from among multiple candidate precoding matrices defined in a codebook. The selection of a particular precoding matrix is generally intended to achieve a desired level of performance, and can be based on any of several factors, such as current system configuration, communication environment, and feedback from an apparatus receiving the transmitted signal.
A common way to use feedback in selecting a precoding matrix is to define the same codebook at both a transmitting apparatus (e.g., a base station) and a receiving apparatus (e.g., a user equipment [UE]), and allow the receiving apparatus to “recommend” a precoding matrix from the codebook—e.g., by transmitting a precoding matrix index (PMI)—based on properties of received signals. For instance, if the receiving apparatus determines that a received signal has relatively low signal-to-noise ratio (SNR), it may recommend replacing a current precoding matrix with a new precoding matrix to increase the SNR.
Under certain circumstances, a system may limit the set of candidate precoding matrices that can be selected from the codebook. In other words, the system may prevent the receiving apparatus from selecting some precoding matrices while allowing it to select others. Such a restriction can be referred to as “codebook subset restriction” or “precoding weight set restriction”, terms that may be used interchangeably in this description.
One way for a system to implement codebook subset restriction is to transmit a “codebook subset restriction bitmap” from a transmitting apparatus to a receiving apparatus. The codebook subset restriction bitmap typically comprises a bit corresponding to each precoding matrix in the codebook, where the value of each bit (e.g., “0” or “1”) indicates to the receiving apparatus whether or not it is restricted from recommending a corresponding one of the precoding matrices.
A general drawback of codebook subset restriction is that it tends to increase signaling overhead. For instance, in some systems, the codebook subset restriction bitmap comprises 64 bits per channel, so a transmitting apparatus may be required to transmit a relatively large amount of information to implement codebook subset restriction for all of its channels. Such transmissions may be an inefficient use of available communication resources.